Query re measuring II input doserates

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>I would like some advice on acceptable kVps for measuring II input
>doserates. We use 20 mm of Al as the beam attenuator, and
>measure the doserate at the II face in ABC mode.
>
>In "Specification, Acceptance Testing and Quality Control of
>Diagnostic X-Ray Imaging Equipment" (Seibert, Narnes & Gould,
>eds), 1994, AAPM, it is noted that for a 3.8 cm (1.5 in) thick Al
>attenuator, the measured doserate is fairly independant of kVp
>ranging from 50 to 110 (p 487). There are further data indicating
>that for 22 mm total Al attenuation, the conversion efficiency varies
>by about 15% between 60 and 110 kVp (p 443).
>
>However, I have also heard it said that for II input doserate
>measurements, the kVp should ideally be kept to around 70 to 80
>kVp, otherwise the doserate measured may be high.
>
>I would appreciate some thoughts on this.
>
>Thanks
>
>John Burrage
>---
>Department of Medical Physics
>Royal Perth Hospital, Perth, Western Australia
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